1. Field of the Invention
The present invention relates to an automatic focus adjustment device and an automatic focus adjustment method to automatically focus on a moving subject.
2. Discussion of Related Art
Automatic focus adjustment methods for automatically adjusting focusing on a moving subject are known. FIG. 10 illustrates focus adjustment using a conventional automatic focus adjustment device. In FIG. 10, the horizontal axis indicates time and the vertical axis indicates, for example, a forwarding amount of a lens based on an infinite position. In FIG. 10, the thick line identifies the locus of the subject image surface (i.e., the locus of an ideal forwarding position of a lens that maintains a moving photographic subject constantly in focus). The thin line identifies the locus of the lens focus position (i.e., the locus of the forwarding amount of the actual lens position).
In general, the automatic focus adjustment device detects a defocus amount (i.e., position deviation) of the subject image surface based on a lens focus position of a phototaking lens assembly. The defocus amount and a sequential change amount of the defocus amount are fed back to a lens forwarding mechanism as control deviations to track the locus of the lens focus position on the locus of the subject image plane.
In this condition, when an actuator (i.e., a release button) is fully depressed at time Tnow, a defocus amount DEFx at exposure time Tex is linearly estimated based on a defocus amount detected before release of the release button and a sequential change amount of the defocus amount. The lens focusing position is changed by DEFx by controlling the lens forwarding mechanism. The lens focus position is then adjusted to a predicted position of the subject image plane at exposure time Tex.
In the conventional automatic focus adjustment devices, however, an overrun condition occurs in the direction of movement of the subject image plane during focus adjustment, as described in FIG. 10. When the defocus amount exceeds a value considered to be in focus at time Tover, the automatic focus adjustment device temporarily stops driving the lens. The lens driving is not resumed until the next distance measurement result is obtained. This stoppage produces a delay and furthermore fails to smoothly adjust the focus position. Furthermore, the repeated driving and stopping of the lens consumes a large amount of power resulting in rapid depletion of the battery. Additionally, during full depression of the release button, a deceleration period is necessary for stopping the lens forwarding mechanism with superior precision at a predicted position of the subject image plane.
Due to the above-stated problems, the desired driving amount of a phototaking lens assembly by the lens forwarding mechanism for a subject moving with a high velocity is not realized during a limited time (e.g., between Tnow to Tex in FIG. 10). As a result, proper focus adjustment at exposure time Tex is impossible.